Several types of circuits used for detecting open load or short circuit condition on a pin exist. Some of those are not able to distinguish between open load and short circuit condition.
U.S. Pat. No. 4,962,350 describes an automatic load condition detection system for detection of a normal or abnormal load condition. The system described distinguishes between short circuit condition and open load condition. A two-phase measurement is used. During a first phase, a load current is supplied to a load from a power source device. In a first phase, a discrimination is made between an abnormal operating condition with the load in an open circuit condition, and another operating condition. If, during the first phase, it is detected that the load is not in an open circuit condition, then during a second phase there is discriminated between a normal operating condition and short circuit condition of the load. If either a short circuit condition or an open circuit condition of the load is detected, then either a short circuit detection signal or an open circuit detection signal representing said condition is outputted from the load condition detection system.
The device described in the above document is not able, once a short circuit condition has been detected, to distinguish between short circuit to battery or short circuit to ground.
In WO 92 21985, a circuit for checking the operating state or status of a sensor is described. The circuit comprises a first testing transistor which blocks when VHPL<Vcontrol, TR1 because of short circuit to ground or interruption, a second testing transistor which blocks when LPL is grounded during short circuit to ground and a third testing transistor which blocks when HPL and LPL have a potential of the electron receiving electrode with respect to the minimum control voltage. Combination of the switching states of the three testing transistors indicates the operating state of the sensor, i.e. distinguishes between operational, short circuit to ground, short circuit to supply voltage and interruption.
In WO 92 21985, distinction can been made between short circuit to ground and short circuit to supply voltage as well. This is done by the combination of the different testing transistors. One disadvantage, however, of the method described in WO 92 21985 is that a resistor is permanently taking current , which results in a continuous energy loss.
U.S. Pat. No. 6,226,602 provides a circuit arrangement and a method with which short circuit to ground or to supply voltage and open load can be distinguished. The method in this document comprises: providing a first circuit and detecting a first state being defined by a short circuit to the supply voltage and providing a second circuit and detecting a second state or a third state, the second state being defined by a short circuit to ground and the third state being defined by interruption of the connection to the supply voltage.
The method in this document is able to distinguish between short circuit to ground, short circuit to supply voltage and interruption of the connection to the supply voltage. The method first determines whether a fault-free or a fault condition is present and then determines whether the fault condition is an interruption of the connection to the supply voltage, a short circuit to ground or a short circuit to supply voltage.
U.S. Pat. No. 5,920,452 describes a method for protecting a circuit from overcurrent condition and for detecting an open load. The method comprises determining if there is a normal operation or an abnormal operation (load is for some reason not present (open load) or shorted to ground or to supply voltage) and providing a microprocessor having a microprocessor terminal configurable as an output for generating a command signal to an electrical load as an input for monitoring the condition of the open load. Furthermore, an output logic signal is generated form the second transistor back to the microprocessor and the first transistor, to latch the first transistor into one of either ON or OFF states under normal load conditions responsive to the receipt of a command signal from the microprocessor. Under abnormal load conditions, the first transistor is maintained in an OFF state upon an overcurrent condition through the first transistor, and a logic signal is maintained to the microprocessor after repeated attempts to switch the first transistor into an OFF state based on an open electrical load condition. In this way, using the method according to U.S. Pat. No. 5,920,452 it is possible to distinguish between open load, short circuit to ground and short circuit to supply voltage.